It is well known in vehicle occupant restraint systems to provide a shoulder belt which has an outboard end mounted on the vehicle pillar by a retractor, and an inboard end releasably connected to the vehicle by a buckle. A guide loop mounted high on the pillar deflects the shoulder belt diagonally across the occupant upper torso. The retractor includes a reel with the belt connected thereto and a winding spring urging belt winding rotation of the reel to pull the belt taut about the occupant torso.
It is well known that the winding effort of the spring pulling the belt taut against the occupant can be perceived as uncomfortable by some occupants. Accordingly it is desirable to design the shoulder belt system to provide sufficient winding bias on the belt to maintain the belt in a properly taut condition about the occupant, and yet the belt not be so taut as to be uncomfortable to the occupant.
In addition, it is desirable that the shoulder belt system comfortably accommodate forward leaning movement of the occupant to reach vehicle controls.
I have discovered that the forces exerted on an occupant by a shoulder belt can be considerably greater when the belt is being extended form the retractor than when the belt is being retracted. For example, FIG. 12 plots the force imposed on the sternum by the shoulder belt, vs. the forward displacement of the upper torso. The plot designated "A" shows that the force exerted on the sternum rises rapidly from 2 newtons to about 9 newtons within the first 20 millimeters of forward motion of the shoulder. This same plot "A" shows that the force on the sternum remains about constant at 9 newtons as sternum displacement continues forwardly, and then, the rearwardly leaning movement of the upper torso causes the force on the sternum to fall off rapidly to about 2 newtons as the belt is being retracted.
I have also studied the subjective reactions of belted occupants and determined that occupants are particularly sensitive to the belt loadings which occur within the first 20 millimeters of forward movement as it is these motions which are most often made involuntarily by the occupant as the occupant breathes or subtly shifts his position to accommodate vehicle decelerations, cornering, etc. Forwardly leaning motions which occur beyond 20 millimeters are more likely to be intentionally made by the occupant and as such the occupant is not subjectively averse to feeling the force of the shoulder belt imposed on his shoulder as he leans forward.
In view of the forgoing I have discovered that it would be desirable to provide a shoulder belt system which could provide a low belt effort on the occupant during forward occupant displacement of about 20 millimeters.
Accordingly, the object feature and advantage of the invention resides in the provision of a auxiliary spring which is interposed in the shoulder belt system between the retractor and the occupant and is effective to permit limited contraction and extension of the belt independent of the winding and unwinding of the belt by the retractor.